The transmissive liquid crystal display (LCD) exhibits a high contrast ratio and good color saturation. However, its power consumption is high due to the need of a backlight. At bright ambient, the display could be washed out completely. On the other hand, a reflective LCD uses ambient light for reading displayed images. Since it does not require a backlight, its power consumption is reduced significantly. However, its contrast ratio is lower and color saturation much inferior to those of the transmission type. At dark ambient, a reflective LCD loses its visibility. Transflective LCDs use a combination of transmissive and reflective modes to provide improvements in image display and power consumption.
Two types of transflective LCDs have been developed: single cell gap (FIG. 1) and double cell gap (FIG. 2).
A single cell transflective LCD is disclosed in U.S. Pat. Nos. 6,281,952 B1 to Okamoto et al.; 6,295,109 B1 to Kubo et al.; 6,330,047 B1 to Kubo et al., commonly assigned to Sharp Kabushiki Kaisha, which use a split-pixel approach, i.e. each pixel is split into reflective (R) and transmissive (T) sub-pixels. Usually, the R and T area ratio is 4:1, in favor of the transmissive display. The transmissive display is used for dark ambient only in order to conserve power.
In the conventional single cell gap approach shown in FIG. 1, the cell gap (d) 100 for R and T modes is the same. The cell gap is optimized for R-mode. As a result, the light transmittance for the T mode is lower than 50% because the light only passes through the LC layer once.
In the conventional double cell gap approach 200 shown in FIG. 2, the transflective LCD has separate transmission and reflection pixels in order to compensate the unmatched liquid crystal retardation. The cell gap is d and 2d for the R and T pixels, respectively. In this approach, both R and T have high light efficiency. However, the T mode has four times slower response time than that of the R mode. Moreover this approach has a complicated structure and fabrication process. Glass etching and indium-tin-oxide (ITO) electrode coating on the transmission region are needed. The cell gap accuracy and uniformity can be poor depending critically on how accurate and uniform the extra thick organic layer is formed. Poor cell gap accuracy and uniformity result in deteriorated LCD performances, such as variations in brightness and color.
U.S. Pat. No. 6,020,941 to Yao-Dong Ma employs switchable liquid crystal materials of two polarities in separate channels, a wall located in an interstice between the separate channels defines a first and a second set of independent cells in the LCD. A first controllable liquid crystal (CLC) material is located in the plurality of independent cells, the first CLC material selectively exhibits an “on” state and an “off” state and has a first polarity when in the “on” state; and a second CLC material located in the plurality of independent cells, the second CLC material selectively exhibits an “on” state and an “off” state and has a second polarity when in the “on” state.
Another cell wall structure is disclosed in U.S. Pat. No. 4,720,173 to Okada et al. and is used to improve the alignment or orientation of the liquid crystal molecules. There remains a need to improve the quality of liquid crystal displays and to provide them at lower costs.